This invention relates generally to the use of magnetic resonance imaging for determining properties of moving fluids such as oxygen saturation of blood, for example, and more particularly the invention relates to determining blood oxygen saturation or other property based on the spin-spin relaxation time (T2) of the fluid.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1225. The basic concepts are described in the lead article, "Introduction to the Principles of NMR," by W. V. House, pp. 1220-1226, which employ computed tomography reconstruction concepts for reconstructing cross-sectional images. A number of two-and three-dimensional imaging methods are described. Medical applications of NMR are discussed by Pykett in "NMR Imaging in Medicine,": Scientific American, May 1982, pp. 78-88, and by Mansfield and Morris, NMR Imaging in Biomedicine, Academic Press, 1982.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. to 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency, .omega..sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z.G.sub.z, on the static uniform field, B.sub.0, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
The spins of excited nuclei have two relaxation times associated therewith. The spin-lattice relaxation time, T1, is equivalent to the recovery time for spins in realigning with the longitudinal magnetization. The spin-spin relaxation time T2, depends on the decay of the transverse component of the magnetization. Both relaxation times are tissue specific.
Thulborn, et al., "Oxygenation Dependence of the Transverse Relaxation Time of Water Proteins in Whole Blood at High Field," Biophysica Acta 714, pp. 265-270, 1982 reported that the oxygen saturation of blood affected the T2 of the blood for in vitro experiments. However, for in vivo applications the motions of the body and/or of blood itself can adversely affect measurements of magnet resonance signals. Further, a more accurate model is required for correlating measurements of T2 and the corresponding values of blood oxygen saturation for experimental conditions used on in vivo measurement, in people.
The present invention is directed to a method and apparatus for determining the T2 relaxation time of vascular blood (in situ) and estimating oxygen saturation in the blood. The invention is applicable in other systems involving moving fluids wherein magnetic resonance signals are indicative of properties of the fluids.